This invention is directed to molecular sieves and a process for their preparation and use. More particularly, this invention is directed to carbonaceous molecular sieves, having high selectivity ratios and high capacity values. The molecular sieves of this invention are produced by the impregnation of carbonaceous substrates with an organic polymer having a molecular weight of at least 400, as determined by vapor phase osmometry, or with an inorganic polymer.
Munzner et al., U.S. Pat. No. 3,962,129, describes a carbon molecular sieve produced by impregnating a porous coke with a low molecular weight organic compound having a boiling point of from 200.degree.-550.degree. C. as measured at atmospheric pressure. (See Column 2, lines 50-57). This impregnation technique effectively reduces the substrate micropores to between 2 to 6 Angstroms, but generates a sieve with reduced capacity.
Carbon molecular sieves have been prepared by externally coating granular activated carbon with from about 20 to 50 weight percent of a partially polymerized solution followed by curing of the polymer and carbonization (charring) of the polymer coating. See: P. L. Walker, Jr., et al., Conference on Industrial Carbon and Graphite, Proceedings (London) 1965, "The Preparation of 4 .ANG. and 5 .ANG. Carbon Molecular Sieves".
Addison, U.S. Pat. No. 2,761,822, describes a finishing process for activated carbon. This process improves the adsorption selectivity, but reduces the internal capacity of the carbon substrate.
It has been discovered that the macropore structure of carbonaceous substrates may be blocked by impregnation with an organic or inorganic polymer. This polymeric block acts to limit access through the macropore structure of the substrate without significantly affecting either the micropore capacity or the microporous diffusion characteristics of the substrate. Thus, selectivity of the substrate is improved without a substantial loss of substrate capacity.